1. Field of the Invention
The invention relates to a bandpass filter having an adjustable bandwidth, comprising a capacitive element arranged in shunt across the input, two series-arranged inductive branches between which a shunted inductive branch is arranged at a central point and a capacitive element arranged in shunt across the output.
Such a filter is used, for example, in the IF amplifier of a receiver for satellite television signals. The bandwidth of the signal is to be adjusted before demodulation to enable this bandwidth to be adapted to the signal-to-noise ratio. This must be done without affecting the gain of the amplifier or the tuning frequency of the filter.
2. Description of the Related Art
If the passband is modified by varying the overvoltages of the industances (in different ratios so as not to change the frequency), the gain also varies. If the passband is modified by changing the coupling, for example, by modifying the value of a shunt element, the tuning frequency varies. In general, it is extremely difficult to vary one parameter without affecting at least one of the other parameters. This is probably the reason which in the prior art has led to the use of two entirely distinct filters used in a switching mode as described in the document JP-Kodai-193416/82.